1. Field of Invention
This application relates generally to radio frequency communications systems, and in particular to compensating carrier wave frequency and phase recovery in such systems.
2. Description of the Related Arts
In a radio frequency communication system, information is transmitted by a transmitter to a receiver via a carrier waveform. The transmitter encodes the information in the carrier waveform by changing the carrier waveform's phase, frequency, and/or amplitude. The receiver down-converts the received signal to remove the carrier waveform, leaving just the modifications that encode the information (under ideal conditions). In a quadrature phase-shift keying (QPSK) or Quadrature Amplitude Modulation (QAM) system, the down-converting also splits the signal into an in-phase component (the “I-channel”) and a quadrature-phase component (the “Q-channel), which is offset from the I-channel by 90 degrees. However, in order to accurately recover (or demodulate) the encoded information, the receiver must first know the frequency and phase of the carrier waveform to establish a baseline upon which the information is encoded. More specifically, this baseline set of information allows the receiver to preserve the fidelity of the transmitted information and maintain the isolation of channels. The process employed to determine this baseline information is referred to as carrier recovery.
In most existing systems, transmitters and receivers each contain independent oscillators used for modulation and demodulation of the carrier signal. As such, the oscillator of the transmitter is not necessarily matched in frequency and phase with the oscillator of the receiver. This mismatch may prevent the receiver from performing accurate demodulation of the carrier signal.
Carrier recovery may be performed a number of different ways. Common techniques include: (a) use of a matched filter centred at the carrier frequency for phase determination, (b) implementation of a simple phase lock loop, and (c) a combination of (a) and (b). However, these techniques require a sufficiently strong carrier signal, and increasing the power dedicated to the carrier reduces the power available for carrier modulation. Increasing the carrier power in order to perform carrier recovery results in less power being available to the data sidebands, and thus lesser efficiency.
Another approach for carrier recovery and demodulation of the carrier signal is the Costas Phase Lock Loop (PLL), also referred to as a Costas Loop. The QPSK Costas Loop uses the fact that the modulation constellation points are always where the magnitude of the I and Q signals are equal. Only the signs are different, so it decodes the quadrant information and develops an error signal proportional to the difference of the magnitudes (keeping the sign information). This error signal is integrated and filtered and then used to correct the receive local oscillator frequency and phase. While this works well for narrow bandwidths, the wideband switching and differential circuits can be difficult to design and keep stable due to the speed of switching required. Further, systems using a Costas Loop may require a priori knowledge regarding the type of encoding used in the signal.